1. Field of the Invention
The present invention concerns carbon fibers suitable to be utilized for electroconductive composite materials, etc.
2. Description of the Prior Art
Since carbon fibers are light in weight, excellent in mechanical strength and satisfactory also in electro-conductivity, they have been utilized in various application uses such as composite materials in combination with metals, plastics or carbon materials. However, since carbon materials are poor in the electroconductivity as compared with metal materials, various studies have been progressed for improving the electroconductivity of the carbon materials and there have been developed intercalation compounds improved with electroconductivity by inserting various molecules, atoms, ions, etc. between the layers of graphite crystals. By the way, if it is intended to obtain carbon fibers of excellent conductivity by utilizing the techniques of such intercalation compounds, since no great development can be obtained for three-dimensional graphites structure for fibers prepared by carbonizing organic fibers and further graphitizing them, it is difficult to incorporate materials between layers. Then, if the processing conditions for forming the intercalation compounds are made severe, texture of the graphite fibers are destructed to damage the mechanical strength or they are powderized, as well as there has been a problem that the thus obtained intercalation compounds are not stable.
On the other hand, it has been known that graphite fibers low electric resistivity can be obtained by preparing graphite fibers through heat treatment of gas phase grown type carbon fibers at 2800.degree.-3000.degree. C. which are formed by thermal decomposition of benzene-hydrogen gas mixture near 1100.degree. C. and then immersing such graphite fibers in fuming nitric acid at 20.degree. C. for more than 24 hours (Proceeding of Electrical Society, vol. 98, No. 5, p249-256, 1978). However, even such fibers can not be practical in that nitric acid is split off at high temperature to make the electric resistance instable.